The investment casting process is used to create metal components, e.g. turbine blades and nozzle vane guides, by pouring molten metal into a ceramic shell of the desired final shape and subsequently removing the ceramic shell.
The process is an evolution of the lost-wax process whereby a component of the size and shape required in metal is manufactured using a wax pattern die into which molten wax is injected. The wax pattern is then dipped in ceramic slurry to create a ceramic shell on the wax pattern. The wax is removed and the shell fired to harden it. The resulting ceramic shell has an open cavity of the size and shape of the final component into which the metal can be poured. The ceramic shell is subsequently removed, either physically and/or chemically.
In order to make a component e.g. an aerofoil blade, with internal cavities e.g. internal cooling channels, a ceramic core is required. This is manufactured separately and is placed inside the wax pattern die prior to wax injection.
After casting the metal in the ceramic shell, the ceramic core is removed e.g. leached with alkaline solution, to leave the hollow metal component.
It is important to locate and support the ceramic core in a fixed relationship within the ceramic shell in order to accurately control and thereby ensure consistency in the resulting wall thickness of the hollow metal component after casting.
Various methods are known for locating and supporting the ceramic core within the ceramic shell. A prior art method is shown in FIG. 1. In this prior art method, pins 1 are inserted into the wax pattern 2 until they are in contact with the ceramic core 3. The pins 1 extend from the wax pattern 2 after insertion. The wax pattern 2 is then encased within a ceramic shell 4 which fixes the pins 1 (and the core 3) relative to the ceramic shell 4. Upon removing the wax pattern 2 (by melting) the pins 1 act to maintain the position of the ceramic core 3 within the empty ceramic shell 4 so that as metal is poured into the ceramic shell 4, the ceramic core 3 retains its fixed relationship within the ceramic shell 4.
The pins 1 may be formed of platinum in which case they melt as the metal is cast into the ceramic shell 4. Alternatively, as described in U.S. Pat. No. 4,986,333B, the pins may be made of recrystallized alumina in which case, they remain within the metal component after casting.
Platinum pins are expensive. The cost of platinum pins is of particular concern when casting around elongated, thin ceramic cores which require a considerable number of pins. Furthermore, because platinum pins melt during the metal casting, they may allow movement of the ceramic core as they melt.
Alumina pins are cheaper and, because they remain within the component after casting, they are better able to minimise movement of the ceramic core. However, as acknowledged in U.S. Pat. No. 4,986,333, when the pins are used in the manufacture of gas turbine components such as turbine blades and guide vanes, the alumina pins tend to exit the components under centrifugal force leaving small apertures in the component. In some circumstances, especially when a high number of alumina pins are used, this may be undesirable as it inevitably leads to changes in the cooling system of the component.
Accordingly, there is a need for a method and an apparatus for locating and supporting a core in a fixed space relationship in a shell mould and maintaining this fixed space relationship in the subsequent casting process for production of a hollow metal casting, which ameliorates the problems associated with the prior art pins.